Intermittent renewable electrical power sources such as wind and photovoltaics (PV) cannot replace a significant fraction of our current fossil fuel-based electrical generation unless the intermittency problem is solved. Fluctuations in renewable source power are generally backed up by natural gas fired “peaker” plants. Inexpensive, reliable energy storage at or near the generation site could render the renewable source dispatchable (e.g. demand-following) and permit the gas peakers to replace baseload coal. It could also permit full utilization of the transmission capacity of power lines from the generation site, permitting supply capacity expansion while deferring the need for transmission capacity expansion.
The advantages of flow batteries are giving them increased attention for grid-scale electrical storage [1]: because all of the reactants and products are stored in tanks outside the electrochemical conversion device, the device itself may be optimized for the required power while the required energy is independently determined by the mass of reactant and the size of storage tanks. This can drive down the storage cost per kWh, which is the single most challenging requirement for grid-scale storage. In contrast, in solid electrode batteries the energy/power ratio (i.e. the peak-power discharge time) does not scale and is inadequate for rendering intermittent renewable power sources dispatchable. Most solid-electrode batteries have peak-power discharge times <<1 hr., whereas rendering PV and wind dispatchable require ˜15 and ˜50 hrs., respectively [2].
The commonly recognized technology options for grid-scale electrical energy storage are summarized in Table 1. Commercial activity with zinc-bromine hybrid flow batteries illustrates the technical feasibility of liquid bromine and hydrobromic acid as reactants. However, by its nature the design—involving Zn plating within the electrochemical conversion device—does not permit flow battery-like energy scaling; it also presents a dendrite-shorting risk [1]. Arguably the most developed flow battery technologies are vanadium redox flow batteries (VRBs) and sodium-sulfur batteries (NaSBs). Costs per kW are comparable, whereas VRBs are considerably more costly on a cost per kWh basis, in part due to the high price of vanadium, which sets a floor on the ultimate cost per kWh of a VRB [3]. The vanadium itself costs around $160/kWh based on recent costs for V2O5 [4]. VRBs do benefit from a longer cycle life, with the ability to be cycled in excess of 10,000 times, whereas NaSBs are typically limited to about 4,500 cycles [3]. For VRBs, costs per kW are likely to move lower, as recent improvements in VRB cell design have led to significantly higher power densities and current densities, with values of 0.55 W/cm2 and 0.9 A/cm2, respectively [5], but these don't help lower the ultimate floor on the cost per kWh. These values, to our knowledge, represent the best performance achieved in VRBs reported to date in the literature. NaSBs have to operate above 300° C. to keep the reactants molten, which sets a floor on their operating costs. Over 100 MW of NaSBs have been installed on the grid in Japan, but this is due to government fiat rather than market forces. NaSBs have the longest duration (energy/power) at ˜7 hrs. VRBs are the subject of aggressive development, whereas NaSBs represent a static target. There is also recent work on the regenerative electrolysis of hydrohalic acid to dihalogen and dihydrogen [6-9], where the halogen is chlorine or bromine. These systems have the potential for lower storage cost per kWh than VRBs due to the lower cost of the chemical reactants.
TABLE 1Energy Storage for the grid. From Dunn et al. [3]; origional source EPRI.TechnologyCapacityPowerDuration% EfficiencyTotal CostCostoptionMaturity(MWh)(MW)(hours)(total cycles)($/kW)($/kWh)CAESDemo250505(>10,000)1950-2150390-430 (above ground)AdvancedDemo3.2-48  1-123.2-4  75-902000-4600625-1150Pb-acid(4500)Na/SCommercial7.217.2753200-4000445-555 (4500)Zn/Br flowDemo5-501-10560-651670-2015340-1350(>10,000)V redoxDemo4-401-10465-703000-3310750-830 (>10,000)Fe/Cr flowR&D414751200-1600300-400 (>10,000)Zn/airR&D5.415.4751750-1900325-350 (4500)Li-ionDemo4-241-102-490-941800-4100900-1700(4500)